why would the people in Jonas's world have made the Sameness get rid of people's ability to see color

The exact nature of Jonas's Capacity to See Beyond is finally revealed in Chapter 12 as the ability to see color, which is a capacity that most of the community has apparently lost by means of an earlier choice. Because of Jonas's discovery of colors such as red, he and The Giver bring the debate about Sameness to the forefront of the novel. Jonas knows instinctively that Sameness is wrong, just as he knew that to suppress the Stirrings was wrong, and he brings up the problem to The Giver repeatedly. The Giver in turn understands Jonas's concerns and notes that ending differences helped people in many ways but also caused them to lose something intangible but significant.